In fabricating a semiconductor device, it may be desirable to include copper as part of the device due to copper's low resistivity and ability to carry high current densities. Unfortunately, the use of copper is somewhat problematic. For instance, attempting to deposit a relatively large amount of copper directly onto a dielectric material results in poor adhesion. This can be particularly troublesome if the underlying surface has a variable topography, such as an insulating surface defining a trench. Assuming that it is desirable to fill the trench with copper, one known method of accomplishing this task is to first line the trench with a seed layer, which provides nucleation sites for the subsequent formation of copper. Known materials for such a seed layer include copper itself or aluminum. One known method of depositing the seed layer is physical vapor deposition (PVD). Such a process, however, not only lines the trench but also deposits the seed layer on surfaces outside of the trench.
Once the seed layer has been deposited, copper is then provided using one of several methods, such as electroplating. In that method, the in-process semiconductor device, which may still be part of an undiced semiconductor wafer, is exposed to a solution containing ions of the metal to be deposited. The wafer serves as the cathode and is connected at its edges to a negative terminal of a power supply. A suitable anode is also exposed to the solution and is connected to a positive terminal of the power supply. The power supply generates an electrical current which flows between the anode and the cathode through the solution. The electrical current causes an electrochemical reaction at the surface of the wafer, resulting in the metal ions in the solution being deposited thereon. Electroless deposition is also another option, wherein deposition occurs in an aqueous medium through an exchange reaction between metal complexes in solution and the particular metal to be coated; and an externally applied electric current is not required.
Regardless of the precise deposition process chosen, it is noteworthy that copper will deposit wherever the seed layer is exposed, including outside of the trenches, where copper is not necessarily desired. Thus, the excess copper must be removed using a process such as chemical-mechanical planarization (CMP). Doing so, however, may require multiple CMP steps depending upon the amount of excess copper to be removed and the presence of other layers, such as a barrier layer. This runs counter to the general desire in the art, which favors a minimum number of process steps, minimal process time, and the minimal use of materials such as CMP slurry and copper. In addition, attempts to remove the copper outside of the trench using CMP risks the phenomenon known as “dishing” concerning the copper remaining in the trench. This undesirable effect is further discussed in U.S. Pat. No. 6,080,656 by Shih et al. (hereinafter Shih).
Shih also proposes a solution to the dishing problem. After depositing a copper seed layer over a dielectric layer defining a trench, Shih proposes depositing a continuous insulating layer over the dielectric layer. Shih then patterns the insulating layer using photolithography in order to remove the portions of insulating material over the trench. Shih suggests using the same photo mask used to define the trench in the dielectric. Ideally, only the portion of the seed layer within the trenches remains exposed as a result of this process. Copper is subsequently electroplated and, because the portion of the seed layer external to the trench is covered by the insulating layer, plating does not occur in that location. Rather, plating is limited to the trenches. The purported result is a relatively easier CMP process that need only remove a small amount of copper, the insulating layer, and the seed layer. However, Shih's process requires that the photolithography step used to pattern the insulating layer be in perfect alignment with the photolithography step used to define the trenches in the dielectric. Otherwise, copper will form outside of the trench and copper formation inside the trench will be hindered. Moreover, assuming that such alignment is achieved, Shih's process still requires additional lithography and etching steps. As discussed above, there is an ever-present desire in the art to minimize the number of process steps needed to fabricate a semiconductor device.
Other fabrication problems occur in another, seemingly unrelated, area of semiconductor device fabrication; namely, plasma-assisted deposition processes such as plasma-enhanced chemical vapor deposition (PECVD). Ideally, generating a plasma as part of the deposition process results in neutral particles that enhance deposition upon the surface of a workpiece. However, even with the enhancement that a plasma process provides, other factors may interfere with deposition. One such factor is the aspect ratio defined by a portion of the surface. The aspect ratio is defined as the depth of a feature divided by the width of a feature. If the surface defines a feature with a high aspect ratio, such as a deep trench with a narrow width, it is theorized that the isotropic flux of neutrals will decrease within the trench, thereby preventing deposition therein. In some circumstances, deposition does not occur at all within the trench.
This non-conformal deposition is an undesirable result in many instances of fabrication. For example, there are efforts in the art to use a plasma process in order to deposit polymer on the sidewalls of a trench. Doing so allows a decrease in a dimension of a feature. As a result, there are efforts by those skilled in the art to change the plasma-enhanced deposition process to allow a more uniform deposition.